Field of the Invention
The invention is based on a method and on an apparatus for finding a fault in a signal path on a printed circuit board on which an electronic device will be applied.
It is already known to test, for example, signal paths applied as conductor tracks on a printed circuit board, by using a TDR measurement (Time Domain Reflectometry). In this case, by laborious manual work, a test signal is passed from a corresponding measuring device onto the signal path and the propagation time of the wave that is generally reflected from the open end of the signal path is measured. The characteristic impedance of the signal path (conduction path) is detected during this measurement. If an interruption or a short circuit then occurs on the signal path, for example, at a soldering point, the propagation time of the reflected wave changes, since the test signal is reflected correspondingly earlier at the faulty soldering point. The propagation time is thus a measure of the location of a fault on the signal path. Such a measuring device is available, for example, in the case of commercially available oscilloscopes (e.g. a sampling oscilloscope with a reflectometry measuring device made by Tektronix).
Generally, this fault cannot be discovered by using a simple resistance measurement, since a poor (“cold”) soldering point cannot be detected in direct current terms because of other disturbing influences such as different contact resistances at contact points or the like. Moreover, the exact fault location cannot be determined by this method, since only the total resistance in an electric circuit can be measured.
There is a similar problem, for example, in the measurement of housed devices having a so-called test board as a printed circuit board. The test board has one or a plurality of contact bases with a multiplicity of contacts via which a device to be tested is contact-connected, so that a measuring device can set the desired test conditions via corresponding conductor tracks (signal paths).
In the case of the present-day trend toward miniaturization, the spacings between the terminal pins of the housed devices and thus also those of the contacts of the contact base become smaller and smaller, so that entirely satisfactory soldering between the contacts with the associated conductor tracks of the printed circuit board becomes more and more difficult. As a result, incomplete or inadequate soldered connections can arise which, from an electrical standpoint, lead to an interruption and may thus represent a reliability risk during the testing of the devices. Since the conductor tracks and the spacings thereof are also very narrow, interruptions or short circuits can also easily arise during the production process, and cannot readily be identified.
This problem has been solved hitherto by manually testing each conductor track and each contact using the abovementioned TDR measurement. When there are hundreds of signal paths on a printed circuit board, this testing is highly time-consuming. In addition, it is easily possible for new faults to creep in again.